The invention relates to a piezoelectric transformer comprising a piezoelectric body which is divided into a primary portion and a secondary portion, both the primary portion and the secondary portion being able to generate and transform piezoelectric vibrations in accordance with an AC Voltage fed to one portion while a transformed voltage can be delivered from the other portion.
One kind of known piezoelectric transformer of this type comprises a piezoelectric body in the form of an oblong plate operated at a resonance frequency characteristic of the length of the plate. This kind of transformer, thus, operates at relatively low frequencies, which limits its power density.
U.S. Pat. No. 3,736,446 discloses an annular piezoelectric transformer. The piezoelectric material is layered by means of through-going electrodes along a portion of the periphery of this transformer, said portion being operated at oscillations in the peripheral direction. The resonance frequency is relatively low for oscillation types along the periphery. As the transformable power is proportional to the frequency, the power transformable by means of this annular transformer is relatively small.
The invention relates to a piezoelectric transformer which is capable of operating effectively in a mode resulting in high frequencies (such as xe2x80x9cthickness modexe2x80x9d) even when it is made of a high power or high Qm piezoelectric material, without the working resonance being disturbed to any substantial extent by overtones from lower modes. In the piezoelectric transformer according to the invention, the piezoelectric body is constituted by a substantially annular body which has been polarized substantially perpendicular to the peripheral direction. An annular piezoelectric body presents three basic modes of vibration, the periphery presenting the lowest frequency and vibrations substantially perpendicular to the peripheral direction, that is, a planar vibration and a thickness vibration.
The thickness vibration (or, expressed more generally, vibration at a resonance frequency of a dimension of a cross-section of the annular body substantially perpendicular to the peripheral direction of the annular body) represents a comparatively high frequency due to the reduced extent of the determining dimension, as the cross-section of the annular body is normally relatively small compared to the outer transverse dimension of the annular body. Thus, the annular transformer of the invention is adapted to operate at a comparatively higher frequency as it operates on a transverse dimension vibration or thickness vibration instead of a peripheral vibration. Moreover, the geometry provides an increased capacity, which together with the increased operation frequency results in a reduced output impedance. Thus the transformer is able to handle increased currents, typically in the range of 0.1 to 5 A.
It is known that each of the primary and the secondary portions of a piezoelectric transformer can be subdivided in sections, and this is also the case for the transformer according to the invention.
The power in the transformer is transferred as a mechanical vibration. The maximum permissible transferred power depends on the fatigue strength of the ceramics. For a predetermined maximum mechanical strain in the ceramics the transferred power is
P=fresxc2x7xcex5xc2x7volxc2x7keff2 
where
P=transferred power
fres=resonance frequency
vol=volume
keff=efficient piezoelectric coupling coefficient corresponding to the existing mode of vibration
xcex5=dielectric permittivity
When two annular transformers are made of the same ceramics and have the same volume, the maximum allowable strain and xcex5 vol are constant. As mentioned above, the resonance frequencies are higher for the thickness vibration than for the peripheral vibration, and as far as conventional piezoelectric ceramics of the power type is concerned, the associated coupling coefficients are 0.3 to 0.35 (k31, peripheral vibration) and 0.40 to 0.45 (kT, thickness vibration), respectively.
The difference in coupling coefficient results in an increased power density of
(0.45/0.35)2=1.65 
and the resonance frequency is typically a factor 10 higher. Totally, the power density is increased by 1.65xc2x710=16.5.